A process of measuring the surface position of a substrate, such as a wafer, while scanning the substrate to obtain a pattern offset (also to be referred to as SPOC (Scanning Pattern Offset Compensation) hereinafter) will be described first. SPOC means scanning a plurality of shots on a process wafer and obtaining a three dimensional pattern within each shot formed by wiring lines, insulators, and the like. A result obtained by averaging measurement data for the plurality of shots is used as pattern offset data for correcting an output from a focus measurement unit in scanning exposure. This allows smooth focusing (wafer movement) without tracking any slight step in a pattern within the shot. In a stationary exposure apparatus, generation of local defocus or offsets can be reduced by obtaining a shot representative plane free from the influence of any projections and recesses of an underlying pattern or scattering. In a scanning exposure apparatus, focus measurement is not performed at one time. A plurality of predetermined focus measurement points within each shot are sequentially measured during exposure, and a stage holding a wafer is driven on the basis of the measurement result. Accordingly, if a focus measurement value greatly varies to an underlying pattern, disturbance unnecessary for the stage is supplied as a position target value, and the synchronization precision in the X and Y directions may be decreased. For this reason, by performing pattern offset correction to suppress a short period of information of projections and recesses, which depends on the pattern of the underlying layer within a shot, both focus control performance and synchronization control performance are improved.
As a surface position detection mechanism for aligning a substrate serving as an object to be exposed with an exposure region image plane onto which a master pattern is projected, for example, there is known a method of providing a vertical position detection system which measures a plurality of points within an exposure region on a substrate and calculating and adjusting the tilt and vertical position of the exposure region from the vertical position information of the plurality of points, as described in Japanese Patent Laid Open Nos. 62-299716 and 2-102518.
As a method of removing a detection error between vertical position detection systems, i.e., so-called focus sensors arranged to measure a plurality of points within an exposure region, there is known a method as described in Japanese Patent Publication No. 6-52707.
As seen in the above-mentioned examples, an optical surface position detection mechanism, which uses a laser, LED, or the like, as a light source, has become the mainstream. In a general optical method, a slit image illuminated by the light source is obliquely projected onto a substrate serving as an object to be detected using a projection imaging optical system. The slit image is formed again on a position detecting element using a light receiving imaging optical system, and the vertical position of the substrate surface serving as the object to be detected is detected as the position of the slit image on the position detecting element.
In the optical method, the substrate surface serving as a surface to be detected must be sufficiently planarized by applying a resist, and must be able to be considered as an optical mirror plane. The optical method aims at increasing the reflectivity on the planarized resist surface and detecting the position of the resist surface by setting the incident angle with respect to the substrate surface to 70° or more.
In a surface position detection mechanism, a position detection system is provided for a plurality of points within an exposure region to cause the exposure region on the substrate to coincide with the imaging plane of a projection system. With this arrangement, the tilt and vertical position of the exposure region are measured. A surface position adjustment mechanism performs surface alignment at a high precision by adjusting the surface position of the substrate on the basis of the measurement result. The surface position adjustment mechanism further performs stable and high precision surface alignment by correcting (suppressing) high frequency components (pattern offset) of the substrate surface shape depending on the pattern of the underlying layer.
In current pattern offset measurement by an exposure apparatus, scan focus measurement is performed by fixing target values for the Z-, ωx-, and ωy-axes on the basis of a global tilt plane determined by a result of global tilt measurement. Extraction of high frequency components, or the like, is performed for the obtained surface position measurement value, thereby obtaining a pattern offset for correcting a measurement value from a focus measurement unit. With this method, a difference between a wafer stage running plane and an approximate curved surface (approximate plane) of the wafer pattern surface is set as the pattern offset of the focus measurement unit. Accordingly, the target coordinate locus of the stage during exposure will trace an approximate plane of an almost smooth curved surface (plane) whose pattern offset is canceled as the best focus plane.
For example, in Japanese Patent Publication No. 6-52707, which pertains to a conventional example, there is disclosed a method of obtaining the least square plane from the focus measurement result of a plurality of points whose pattern offsets are subtracted, aligning a region to be exposed so as to conform to the plane, and performing exposure.
Along with miniaturization in transfer pattern, the diameter of a projection lens of an exposure apparatus increases. For this reason, the depth of focus of a pattern imaging plane is generally 0.3 to 0.4 μm with respect to a target line width of 0.11 μm. An error, which can be assigned to focusing of an exposure pattern onto a wafer becomes 30 to 40 nm. To focus a region to be exposed across the shot region within the depth, LTV (Local Thickness Variation) of a process wafer must be restricted. This forces disadvantageous conditions in terms of the wafer cost and process management. Thus, there has been a demand for a method of performing high precision focus and exposure processing without strictly restricting the LTV of the wafer.
According to a scanning exposure method using a slit scan, by aligning a substrate to be exposed with the local shape of a shot to be exposed being scanned, a focus margin can be ensured more than a stationary collective exposure method. As a conventional example, there is disclosed a method of supposing a wafer shape as a curved surface represented by a polynomial function and obtaining the pattern offset by subtracting the curved surface from a substrate surface position measurement value. However, in a scanning exposure apparatus, it is difficult to eliminate a tracking deviation with respect to a high-degree wafer surface shape, such as a ramp waveform, in discrete focus measurement and tracking control, only by performing pattern offset correction for a measurement value from a focus sensor.